(1) Field of the Invention
This invention relates to a communication system and, more particularly, to a communication system for performing UWB communication.
(2) Description of the Related Art
Currently, the operation speed of CPUs used in electronics has progressively increased. If the operation speed (frequency) of CPUs used in electronics for radio communication becomes approximately equal to a radio communication frequency, then they will interfere with each other. Accordingly, electronics for radio communication must use radio signals at higher frequencies.
The frequency bands of 3.1 to 10.6 GHz (microwave) and 22 to 29 GHz (sub-millimeter wave) are allocated for ultrawide band (UWB) communication and probing in which a bandwidth ratio (bandwidth/center frequency) is higher than or equal to 20% or in which a bandwidth wider than or equal to 500 MHz is used. The UWB techniques will also be used in a milliwave band in the future.
A bandwidth ratio is high in the microwave band. This enables communication in which the hopping of the time when a single cycle pulse occurs is performed without using a carrier wave. A bandwidth to center frequency ratio is low in the sub-millimeter wave or millimeter wave band compared to microwave UWB band, so a wave train of several to several hundred waves can be used instead of a single cycle pulse in the microwave UWB monocycle system.
FIG. 27 is a block diagram of a UWB transmitter for performing direct sequence spread spectrum communication. A code spreader 142 spreads data to be transmitted by the use of a spreading code outputted from a code generator 141 and sends it to a waveform generator 143. The waveform generator 143 generates a single cycle pulse or a burst waveform on the basis of the spread data to be transmitted. A band pass filter (BPF) 144 takes only a predetermined band from the single cycle pulse or the burst waveform. The predetermined band is transmitted from an antenna 145.
FIG. 28 is a block diagram of a UWB receiver for performing direct sequence spread spectrum communication. Only a permissible band of the UWB signal received by an antenna 151 is outputted to a pulse correlator 153 via a BPF 152. On the other hand, a code spreader 155 generates a spreading signal from a code generated by a code generator 154. A waveform generator 156 generates a received waveform template corresponding to the spreading signal. The pulse correlator 153 detects a correlation between the received waveform template and the received signal. (With binary phase shift keying (BPSK), there is a non-inverted or inverted correlation between the template and the received signal over the entire length of the spreading code. Therefore, after the pulse correlation detection a positive or negative correlation signal is obtained by performing integration over each code interval.) A pulse train integrator 157 calculates an integration value for the received signal in each code interval. A comparator 158 takes demodulated data on the basis of whether the integration value is positive or negative.
The code spreaders and the waveform generators in the transmitter and the receiver must operate at a clock frequency given by (data transmission rate×spreading code length/number of modulated bits). For example, if a data transfer rate is 500 Mbps, spreading code length is 64 bits, and the number of modulated bits is one, then the code spreaders and the waveform generators must operate at a clock frequency of 3.2 GHz.
An oscillation circuit capable of generating multiphase clocks, such as two-phase clocks, which have a certain phase difference and a stable frequency and in which phase noise is low without dividing a high source oscillation frequency or using many phase shifters is disclosed (see, for example, Japanese Unexamined Patent Publication No. 2002-208817, paragraph nos. [0011]-[0021] and FIGS. 1-4).